Spring brake actuators of the type disclosed herein are used in combination with pneumatic brake systems on heavy vehicles including for example trucks, buses and trailers. The considerable inertial mass of these heavy-duty vehicles in combination with the high speeds at which they travel requires a braking system which responds rapidly with substantial braking power. A tandem spring brake actuator of the type disclosed in this application generally includes an H-shaped central housing member or flangecase which defines an emergency chamber on one side and a service or non-pressure chamber on the opposite side, and a power spring located in the emergency chamber which is biased against a piston which drives a push rod operably connected to the vehicle brake. When the pneumatic pressure in the spring chamber falls below a predetermined pressure or where the vehicle pneumatic pressure fails, the power spring drives the push rod to actuate the brakes, thus serving as an emergency brake for the vehicle. When the vehicle is turned off, the spring chamber also acts as a parking brake. The service chamber generally includes a push rod operable connected to the braking system of the vehicle and the push rod in the emergency chamber and the vehicle pneumatic brake system biases a flexible diaphragm in the service chamber to actuate the brakes of the vehicle. Where the spring brake actuator includes both an emergency chamber and a service chamber in tandem, the spring brake actuator is generally referred to as a "piggyback" spring brake actuator. There are generally two types of piggyback spring brake actuators. In the piston-type, the piston in the emergency chamber includes an annular seal at its outer periphery which seals against the inside surface of the cylinder case of the emergency chamber. In a diaphragm-type, commonly referred to as a dual diaphragm spring brake actuator, the emergency chamber includes a flexible diaphragm normally secured between opposed flanges of the central housing member or flangecase and the cover or head.
In certain applications, the emergency chamber is separated physically from the service chamber and the improved brake actuator of this invention may be utilized for piggyback spring brake actuators or brake actuators where the emergency chamber is separated from the service chamber. In a typical dual diaphragm spring brake actuator, the central housing member or flangecase is formed of aluminum, such as SAE 360 aluminum, and the head or cover is formed from steel, typically SAE 1008 or 1010 low carbon steel; however, the central housing member may also be formed of steel.
Because the power spring utilized in spring brake actuators is very powerful, the head of the emergency chamber must be securely attached to the housing member. In the past, the head of a dual diaphragm spring brake actuator was retained to the housing or flangecase by a bolted clamp band. In the piston-type spring brake actuator, the head is secured to the flangecase by a split metal retaining ring. More recently, various tamper-resistant brake actuators have been introduced, by the assignee of the present invention, wherein the head of the emergency chamber is permanently attached to the housing member by inelastically deforming a skirt or lip generally on the head around a radial flange of the housing member as disclosed, for example, in U.S. Pat. No. 4,960,036 assigned to the assignee of this application.
Spring brake actuators are mounted on the underside of a vehicle. Where the space available for mounting the spring brake actuator is at a premium, it is desirable to reduce the overall envelope of the spring brake actuator by eliminating the radial flanges on the housing member and the cup-shaped head. This was accomplished by forming a radial groove in the housing member and deforming the open end of the cover, which is telescopically received over the end of the housing member, into a radial groove in the housing member as disclosed in U.S. Pat. No. 5,676,036, also assigned to the assignee of the present application. The cover is then sealed to the central housing member by an annular resilient seal which is resiliently biased against the inner surface of the cover. If the cover rotates relative to the housing member, the annular seal may tear, resulting in leakage around the seal. It is also important where the brake actuator includes a breather tube to maintain the factory set alignment of the breather tube hole in the head or cover to the ports in the case. Thus, there are specific torque (antirotation) requirements in the specifications for brake actuators. Further, even where the housing is not rotated, leakage may occur around the seal because the annular seal may not be pressed tightly against the interior surface of the cup-shaped cover, particularly under extreme temperature conditions. Thus, a primary object of this invention is to prevent rotation of the cup-shaped cover relative to the housing member and improve sealing of the spring brake actuator.